A U.S. Central Command official says an Iraqi and Kurdish military force is being prepared to recapture the city of Mosul from Islamic State fighters, likely in April or May.

The official, who spoke on the condition of anonymity, said the force would amount to about 20,000 to 25,000 troops, while Mosul is currently being held by an estimated 2,000 Islamic State fighters.

The official also said if the Iraqi forces are not ready at that time, the coalition has not “closed the door” on launching the Mosul attack later in the year. The radical Islamist group overtook the city last June.

Five Iraqi Army brigades are expected to participate in the attack force, according to the official, along with three smaller brigades for a reserve force. He said three Peshmerga brigades would help contain Islamic State fighters from the north and west of the city, and a so-called “Mosul fighting force” comprised of mostly former Mosul police would also participate.

The official said the U.S. has not yet made a decision about whether small numbers of U.S. military advisers will be needed on the ground near Mosul to direct air support.

Military Decline

The official said the Islamic State is “in decline” militarily. He said Iraqi and Kurdish forces have retaken about 700-800 square kilometers of territory the IS militants once controlled, adding that the group’s ability to govern is also very limited.

“Even though we are not physically in Syria, he’s [IS] having to make decisions everyday about what he can and cannot do,” the official said. “In total, our effects are outpacing his ability to regenerate.”

The coalition has conducted 2,477 air strikes to take out IS forces and equipment as of February 17, according to a source within the Pentagon.

Training fighters

Four nations have offered training sites for Syrian rebels to learn techniques to fight the Islamic State threat. A technical agreement between the United States and Turkey concerning the training facility on Turkish territory was signed Thursday.

The Central Command official says a training site in Jordan is “ready to go,” while another in Saudi Arabia will take between 30-90 days to prepare. Qatar has offered a training site, he said, but it is months away from being ready to host troops.

U.S. forces are training Iraqis in five training site across Iraq, with about 2,000 program graduates and about 3,200 currently in training.